In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor and fan. In a more complex version of the gas turbine engine, the compressor and a high pressure turbine are mounted on one shaft, and the fan and low pressure turbine are mounted on a separate shaft. In any event, the hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion gas temperature. The maximum temperature of the combustion gas is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1900-2150° F.
A number of techniques have been employed to increase the operating temperatures beyond the ordinary capability of the nickel-base superalloys that are the preferred materials of construction. In one approach, a protective structure is applied to the surface of the article. Environmental coatings of aluminum-rich alloys are widely used. The upper surface of the environmental coating oxidizes to a protective aluminum oxide scale. Ceramic thermal barrier coatings may also be applied directly to the substrate or, more preferably, overlying the aluminum-rich coating layer. While operable, environmental coatings have maximum-temperature limitations. Ceramic thermal barrier coatings are subject to failure by impact and thermal cycling damage.
Accordingly, there is a need for an improved approach to the protection of articles and their surfaces in the extreme conditions of operation of the components of gas turbine engines. The present invention fulfills this need, and further provides related advantages.